Magnetic ink printing method

ABSTRACT

A method of printing wherein magnetic particles dispersed in an ink are selectively orientated prior to printing onto a document where the particles are aligned on a carrier while the ink is in a low viscous state and the printing is effected when the ink is in a high viscous state. The method by which the viscosity is changed is the selective application of heat.

The present invention relates to printing and is especially related toprinting involving the use of inks containing magnetic particles.

Our British Pat. No. 1331604 relates to the recording of information,especially security information, onto cards having magnetic layersthereon. These cards, which may be security or credit cards, are given a"magnetic watermark" by orientating preselected areas of a coatingconsisting of acicular magnetic particles in a binder, while the coatingis in a liquid state and then causing the coating to solidify. A similarprocess can also be applied to documents, bank notes, cheques or similaritems.

The magnetic watermark patterns caanot be erased by magnetic means sincethey comprise physically structured differences in the magnetic layerswhich remain even after demagnetization. They are read in verifyingequipment which includes a strong permanent magnet. This magnet isstrong enough to saturate the magnetic layers, thus destroying anyconventionally recorded magnetic patterns thereon, which may beforgeries. However, the saturation of the layers also develops aremanence pattern which represents any differences in remanence betweenregions of different orientation. This remanence pattern is sensed by amagnetic read head.

Magnetic inks have been used in printing including, for example, digits,characters or artistic designs, on cheques or bank notes. The magneticink used may consist of acicular magnetic particles, such as magnetite,in a fluid medium, while the magnetic coating usually consists of γFe₂O₃, CrO₂ or similar material dispersed in a vehicle comprising binders,plasticisers etc.

When such magnetic inks are applied, by a printing process, to fibrous,absorbent surfaces such as common paper or card various problems arise.The most important of these are:

1. The acicular particles become lodged in the fibrous structure of thesupport (i.e. paper or card) and their mobility is greatly reduced thushindering alignment of the magnetic particles; and

2. The rapid leaching of the solvents from the ink by absorption intothe support further decreases the ability of the particles to respond tothe applied orientation magnetic field. These factors restrict the abovementioned layer coating process to applications involving paper or cardshaving a "highly finished" surface, wherein the nature of the paperallows substantially full orientation of the acicular particles in amagnetic field.

It is an object of the present invention to reduce the effects of theaforementioned problems.

According to the invention there is provided a method of printing on asurface with an ink including acicular magnetic particles so that theauthenticity of the printing can be verified, wherein a pattern isformed on a carrier with the ink in the wet state, the particles aresubjected to a magnetic aligning process while the ink is on the carrierand the wet ink is transferred to said surface, and the transfer thusbeing affected with the particles already substantially aligned.

The magnetic watermark may be in any sutable design including, forexample, digits, characters or artistic designs printed on cheques orbank notes.

In British Pat. No. 1,183,479 to Pollock a method is disclosed oforientating magnetic particles in a liquid prior to the deposition ofsaid liquid onto a tape. However, the present invention is distinguishedfrom this in that Pollock applies a magnetic aligning field to the inkbefore the ink is applied to the carrier, whereas in the presentinvention the aligning field is not applied to the ink until after theink has been applied to the carrier.

In order that the invention may be clearly understood and readilycarried into effect embodiments will now be described by way of exampleonly, with reference to the accompanying drawings of which:

FIG. 1(a), (b) and (c) shows a simple arrangement according to theinvention in perspective and cross-sectional views respectively,

FIG. 2 shows one example of a printed design,

FIG. 3 shows a document carrying a portion printed according to theinvention, and

FIG. 4 shows a different arrangement for effecting one aspect of theinvention.

Since the magnetic particles are aligned in the ink prior to printingthere are problems relating to the ink to be considered. These problemsprimarily arise because of the separate requirements that need to besatisfied by the ink and are:

1. the necessity of the ink to allow good alignment of the magneticparticles -- this requires a low viscosity for the ink.

2. The ink has to give a good quality print and this requires a highviscosity ink, and

3. That on release of the platen from the paper the ink is not drawn upinto threads which fall back onto the wet layer and disturb thepre-orientation and this requires what is known in the art as a shortink, for use in plate printing by the transfer process where all the inkis transferred from a platen to the paper surface.

Since these requirements are essentially mutually incompatible it is notapparently possible to incorporate them into a single formulation ofink. A method of overcoming these problems used in the present inventionis the preheating of the ink to reduce its viscosity to allow theorientation of the magnetic particles whereon on cooling the ink assumesa form acceptable for good printing.

A typical formulation for the ink used in the present invention is:

    ______________________________________                                        CrO.sub.2         28%      Magnetic Pigment                                   Coated CaCO.sub.3 33%      Extender                                           MgCO.sub.3         4%      Filler                                             Rosin modified phenolic resin                                                                   34.5%    Varnish                                            in raw linseed oil                                                            6% Cobalt Nuosyl   0.5%    Drier                                              ______________________________________                                    

Referring now to the drawings FIG. 1 shows a simple arrangement forworking the invention. An engraved cell 1 is cut in a non-magneticplaten 2. Suitable dimensions used are 0.5 inches long, 0.125 inchesbroad and 0.01 inches deep leaving about 0.001 inches at the bottom.These dimensions are not critical, however, and any cell of similardimensions such as used in printing is applicable. The cell is thenfilled with ink according to the abovementioned formulation, any excessbeing wiped from the platen, and is heated (by means not shown) to 55°C, this temperature being one to provide a low viscosity for the ink atthe same time as being below the Curie Point for chromium dioxide andadditionally not affecting the chemical nature of the ink in an adversemanner. The heated ink and platen is then placed in a magnetic fieldprovided by an electromagnet 3 fed from a source (not shown) throughwinding leads 4. This magnet field is arranged to orientatesubstantially all the magnetic particles to lie along the breadth of thecell. Theoretical considerations show that complete alignment of themagnetic particles in the direction of the applied field is notpossible. However, to achieve optimum alignment a field of 3K gauss isapplied by the electromagnet 3 reduced to 2K gauss to preventoverheating of the electromagnet. This field is applied in the order of20 seconds to allow optimum alignment to occur. The field is removed anda second magnet 5 is passed slowly along the length of the cell 1. Themagnetic field of this magnet is controlled through winding leads 6 bymeans not shown and is arranged to be on or off. In the on situation afield of some 1.5K gauss is generated in the gap of the magnet 5 andthis field causes alignment of particles along the length of the cell.

FIG. 2 shows an arrangement of bars which represents a number, characteror other design. The blocks of bars ABCD are engraved in the plate andas in the above procedure the alignment of magnetic particles isachieved in each block in the direction shown by the arrows. An improvedmethod of aligning the particles for an example such as is shown in FIG.2 will be described later with reference to FIG. 4.

The output signal derived from the verifying equipment after D.C.saturation depends upon the remanence of the element, which is relatedto the depth of the engraving and the direction of the orientation.Blocks A and C, in FIG. 2, have maximum remanence, due to orientation,in the left-right direction while the blocks B and D would have aminimum in that direction. The converse is the case for the up-downdirection.

FIG. 3 shows a document 7, with a selected portion printed according tothe invention. The "magnetic watermark" can conveniently be printed sothat it is optically indistinguishable from the rest of the design. Inthis example, for instance, the downstroke of the "T" in "TRAVELLER'S"comprises a magnetic watermark but it is not visibly distinct from theother letters in the writing.

In large scale printing operations where a few hundred or more documentsneed to be printed the apparatus as described with reference to FIG. 1is not convenient and different systems have to be used. In theseprocesses large platens with a number of cells and/or a pattern engravedon the surface are used as also could be rollers and other similarprinting techniques. In these the ink can be either pre-heated beforeapplication to the platen or the platen itself can be heated (as is donenow in printing in certain circumstances), the application of the ink tothe paper providing enough cooling for the ink to return to its unheatedhigh viscosity state.

The length of time required to align the particles in the magnetic fieldis an important factor and an example of an improved method of alignmentis shown in FIG. 4. The magnetic particles in an ink in a cell 1 asbefore are aligned by a first magnetic field (not shown) to liesubstantially in the direction given by the arrow 8. A multitrackrecording head 9 is then passed slowly beneath the cell 1 in thedirection of the arrow 12. In practise this head 9 would include 40separate recording heads 10 but only five have been shown for the sakeof clarity. Each of these recording heads 10 is then fed by D.C. signalsthrough leads 11 and these D.C. signals can be such as to energise thehead 10 or to leave it in the off position. In the figure the heads arerespectively ON, OFF, ON, ON, OFF from the top downwards. When this headis passed slowly behind the cell 1 then those magnetic particles lyingabove the ON recording heads will be aligned in the direction of thearrow 12 the other particles being unchanged. The sizes of fields usedare similar to those given for magnet 5 above and a passage time oftwenty seconds is needed for optimum alignment. Apart from providing afaster method of orientation than that of FIG. 1 there is the additionaladvantage that the transition regions between different alignments ofparticles (i.e. left-right or up-down) are much sharper. This isimportant in allowing for carrier verification. The multitrack head can,of course, being placed beneath the cell and the fields merely switchedon and off without movement.

Although the above invention has been described with respect to aspecific formulation of ink there are many similar formulations wellknown in the art. The magnetic pigment, in this case chromium dioxide,could equally be magnetite or similar material. Improvements can be madeby using longer particles than chromium dioxide, which is about 0.5microns long, to get a greater magnetic moment and particles of theorder of 2-3 microns are particularly useful. The magnetization can befurther improved by using metal particles such as iron fillings of thecorrect dimensions i.e. acicular. In all examples, however, the aligningfields need to be at least twice the coercive force of the magneticparticles.

Similarly changes in viscosity of the ink have been achieved by heatmethods, other methods which can be employed include thixotropy, (wellknown in non-drip paints), and methods using solvent evaporation, atpresent used in the production of magnetic tape coatings.

What I claim is:
 1. A method of printing on a surface with an inkincluding acicular magnetic particles so that the authenticity of theprinting can be verified, wherein a pattern is formed on a carrier withthe ink in the wet state, the particles are subjected to a magneticaligning process while mobile in the ink on the carrier, the wet ink istransferred to said surface and dried to immobilise the particles intheir alignment, the transfer thus being effected with the particlesalready substantially aligned.
 2. A method of printing according toclaim 1 in which different areas of the pattern are selectively alignedand are in a plurality of different directions.
 3. A method of printingaccording to claim 1 in which the aligning process is carried out whilethe ink is in a low viscosity condition and the viscosity is increasedbefore transfer occurs.
 4. A method of printing according to claim 1wherein the aligning process is carried out while the ink is heated toreduce its viscosity and is transferred after cooling.
 5. A method ofprinting according to claim 1 in which the transfer is affected in sucha way that substantially all the ink of such a pattern is transferredfrom the carrier to the surface.
 6. A method of printing according toclaim 1 wherein said magnetic aligning process comprises twoperpendicular magnetic fields applied consecutively to the ink on thecarrier.
 7. A method of printing according to claim 6 wherein the secondapplied magnetic field is varied at a predetermined rate so to provideselective orientation of said magnetic particles.
 8. A method ofprinting according to claim 7 wherein said second magnetic field isprovided by a single transducing head being passed along a predeterminedpath adjacent to said carrier wherein the magnetic field produced bysaid transducing head is varied over said path.
 9. A method of printingaccording to claim 1 wherein said magnetic aligning process comprises afirst magnetic field applied in a first direction, to substantiallyalign the magnetic particles in a selected area and a plurality ofdiscrete magnetic fields applied in a second direction at separatesections within said selected area.
 10. A method of printing accordingto claim 9 wherein said plurality of discrete magnetic fields isprovided by a multitrack transducing head.
 11. A method of printingaccording to claim 1 wherein said magnetic aligning process utilizesmagnetic fields at least twice that of the coercivity of the magneticparticles.
 12. A method of printing according to claim 1 wherein saidacicular magnetic particles are chromium dioxide.
 13. A method ofprinting according to claim 1 including also printing on said surfacewith an ink not including aligned acicular magnetic particles a designof a selected portion of which is formed by said pattern on transfer tothe surface.